Biomimetic reductive amination under the continuous-flow reaction conditions

Article abstract of DOI:10.1016/j.jfluchem.2009.10.013

This study present a full account of continuous-flow reaction conditions for biomimetic reductive amination of fluorinated carbonyl compounds to corresponding amines and amino acids of biomedical importance. We demonstrate that simple silica-adsorbed DBU can be used as efficient catalysts for on-column 1,3-proton shift reaction, a key transformation in the biomimetic reductive amination process. This new on-column process features operationally convenient conditions, higher chemical yields, enantioselectivity and purity of the corresponding products as compared with traditional in-flask reactions. Moreover the removal of base-catalyst, the most delicate problem of the in-flask reactions, is not an issue in the on-column process, as the silica-adsorbed DBU or polymer-bound guanidine remains on the column and can be reused. This feature renders the overall process substantially more economical and synthetically efficient, in particular, for large-scale synthesis of the corresponding fluorinated amines and amino acids target.

Full text of DOI:10.1016/j.jfluchem.2009.10.013

Journal of Fluorine Chemistry 131 (2010) 261–265
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Biomimetic reductive amination under the continuous-flow reaction conditions
Vadim A. Soloshonok a,b,c, , Hector T. Catt a,b, Taizo Ono a,b
*
a Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of the Ukraine, Murmanska Street, Kyiv 94 02660, Ukraine
b National Institute of Advanced Industrial Science and Technology (AIST), 2266 Anagahora, Shimoshidami, Moriyama-ku, Nagoya, Aichi 463-8560, Japan
c Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK 73019, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
This study present a full account of continuous-flow reaction conditions for biomimetic reductive
amination of fluorinated carbonyl compounds to corresponding amines and amino acids of biomedical
importance. We demonstrate that simple silica-adsorbed DBU can be used as efficient catalysts for on-
column 1,3-proton shift reaction, a key transformation in the biomimetic reductive amination process.
This new on-column process features operationally convenient conditions, higher chemical yields,
enantioselectivity and purity of the corresponding products as compared with traditional in-flask
reactions. Moreover the removal of base-catalyst, the most delicate problem of the in-flask reactions, is
not an issue in the on-column process, as the silica-adsorbed DBU or polymer-bound guanidine remains
on the column and can be reused. This feature renders the overall process substantially more economical
and synthetically efficient, in particular, for large-scale synthesis of the corresponding fluorinated
amines and amino acids target.
Received 31 July 2009
Received in revised form 25 October 2009
Accepted 28 October 2009
Available online 13 November 2009
Keywords:
Transamination
1,3-Proton shift reaction
On-column reactions
Continuous-flow reactions
Biomimetic reductive amination
Fluorine-containing amines and amino
acids
ß 2009 Elsevier B.V. All rights reserved.
Operationally convenient conditions
Asymmetric synthesis
Dedicated to the memory of
Professor L. M. Yagupolskii, the
founder of Fluorine Chemistry
in Ukraine.
The development of bio-inspired synthetic methodology is
currently considered as the most promising direction in modern
organic chemistry [1]. The overall advantage of the biomimetic
reactions over traditional chemical methods is that this approach
offers a ‘‘greener’’, environmentally benign and operationally
convenient [2] methodological option for preparation of target
organic compounds. Among various biological processes, the
enzymatic cofactor pyridoxal 50-phosphate-catalyzed transamina-
tion [3] (Scheme 1) and related reactions inspired many organic
chemists to investigate its mechanistic role [4] and develop the
corresponding chemical models for practical synthetic applica-
tions [5].
The key step in chemical adaptation of the biological transami-
nation is the control over the equilibrium between imines 3 and 4.
However, it is well known that this type of azomethine–azomethine
isomerization (1,3-proton shift), due to the usually low C–H acidity
of aliphatic imines, requires application of relatively strong base-
catalyst under impractically harsh reaction conditions [5k,l]. From
the theoretical stand point, the required isomerization as well as the
position of the equilibrium can be facilitated and controlled by the
designed combination of highly electrophilic carbonyl compound 2
and nucleophilic amine 1 or, vice versa, electrophilic amine 1 and
nucleophilic carbonyl compound 2. Since the latter combination is
rather a curiosity, the practical synthetic models mimicking the
biological transamination has been achieved with application of
highly electrophilic carbonyl compounds 2 for oxidative deamina-
tion of amino compounds 1. Professor Corey’s group was first use
this biomimetic principle for oxidative deamination, as a key
reaction step in several reported by them total syntheses of natural
products[6]. Othergroups havedevelopedandintroducedaseriesof
truly efficient reagents for practical and generalized transformation
of amines to the corresponding carbonyl compounds via this
biomimetic intramolecular oxidation–reduction reaction [7]. The
opposite transformation, the biomimetic reductive amination of
carbonyl compounds, turned out to be more difficult to realize, as in
this case the increasing nucleophilicity of potential amine-reagent is
mirrored by the decreasing C–H acidity, thus requiring stronger
base-catalysts and harsher reaction conditions. Nevertheless, a
number of successful examples have been reported [8], in particular
* Corresponding author at: Department of Chemistry and Biochemistry,
University of Oklahoma, Norman, OK 73019, United States.
E-mail address: vadim@ou.edu (V.A. Soloshonok).
0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2009.10.013

262
V.A. Soloshonok et al. / Journal of Fluorine Chemistry 131 (2010) 261–265
Scheme 1. Biomimetic oxidative deamination (from 1 to 6) and reductive amination (from 6 to 1).
for asymmetric transamination of
[5a,b,i,j,9].
a-keto to
a-amino acids
[17]. Catalytic asymmetric synthesis of amines 12 using cincho-
nidine derived bases was also demonstrated [18].
Due to the strong electron-withdrawing nature of fluorine,
fluorinated carbonyl compounds are generally highly electrophilic
derivatives and therefore, one would envision that their biomi-
metic reductive amination with alkyl amines might hold promising
synthetic potential. In 1988 [10], our group in Kiev, led by
Professors V. P. Kukhar and Y. L. Yagupolskii, was first to
demonstrate the practicality and generality of this approach for
preparation of various fluorinated amino compounds of biomedi-
cal importance. Scheme 2 shows the up-to-date synthetic scope of
the biomimetic reductive amination of fluorinated carbonyl
compounds. Thus, the most straightforward application of this
methodology is preparation of fluoroalkylamines 8 by transami-
nation of the corresponding aldehydes and ketones with benzy-
lamine or picolinamines in the presence of triethylamine (TEA)
[11]. Application of benzylamine for transamination of perfluor-
It should be noted that this methodology is not limited to the
transamination of carbonyl compounds containing fluoroalkyl
group directly bonded to the carbonyl function. Its potential for
preparation of benzylamines 13 containing, for instance, trifluor-
omethyl groups of the phenyl ring has been recently demonstrated
[19].
While the high practical value of this biomimetic reductive
amination methodology for preparation of fluorinated amines and
amino acids, was appreciated by the synthetic community [20] its
major advantage over the conventional reduction amination
methods, the intramolecular reduction/oxidation process, has
never been synthetically explored. We have envisioned that since
this biomimetic process does not require the use any external
reducing reagents; a conceptually new continuous-flow, on-
column, reductive amination process can be realized. Here we
report a full account [21] of our studies on the development of
continuous-flow reaction conditions for biomimetic reductive
amination of fluorinated carbonyl compounds.
oalkyl
substantial haloform reaction-type decomposition of the starting
keto esters [12]. On the other hand, more sterically bulky rac-
(phenyl)ethylamine allowed preparation of -amino acids 9 in
good isolated yields [13]. Synthesis of -amino acids 10, via
a-keto carboxylic esters was found to be complicated due to
a
-
a
Set-up of a suitable process for a continuous-flow reaction using
the principle of biomimetic transamination is very simple (Scheme
3) and can be readily assembled and used in any organic chemistry
laboratory.
The only issues which can be determined by experiments are
the relative proportions between the obviously needed three zones
designed for protection, reaction and purification (to remove
b
transamination with benzylamine can be cleanly conducted at
elevated temperatures and in the presence of stronger than TEA
bases, such as DBU or guanidine [14]. Of particular interest is a
unique example of double biomimetic transamination of fluori-
nated carboxylic acids to
a,a-dihydroperfluoroalkylamines 11. In
this case both -protons of starting benzylamine are used as
a
completely any trace of the base-catalyst). To this end, a
‘‘reducing reagents’’. Taking into account that the whole one-pot
procedure takes seven separate steps, the obtained yields of >90%
were simply remarkable [15]. Asymmetric version of this
methodology was developed using inexpensive enantiomers of
conventional Pyrex-glass column was packed about 3/4 of height
with a regular silica gel (hexanes), followed by addition, on top of
the column, of the calculated amount of DBU (0.25 wt% of the
whole amount of silica gel) in a solution of dichloromethane. This
step should be done carefully, allowing the solution of DBU to
slowly percolate down to the surface and thus generate the
reaction zone. An additional amount (1/4 of the whole amount) of
silica gel should be charged carefully onto the column, as
protection zone. According to experimental data, the DBU
penetrates into the silica gel to occupy about 1/4–1/5 of the
a
-(phenyl)ethylamine, as transaminating reagent. In most cases,
due to the lower C–H acidity of the chiral amine as compared with
that of benzylamine, application of DBU was required [16]. It is
interesting to mention that this reductive amination represent a
quite rare case of highly (>90% ee) stereoselective proton transfer
from a more to a less configurationally stable stereogenic center
Scheme 2. Synthetic scope of biomimetic reductive amination of fluorinated carbonyl compounds.

V.A. Soloshonok et al. / Journal of Fluorine Chemistry 131 (2010) 261–265
263
Table 1
Yields for isomerization of 14a–n to 15a–n under the conditions of continuous-flow
reaction.
14
R
RF
Yield of 15
On-column
In-flask (lit. data)
a
b
c
d
e
f
H
CF3
>98
95
87
86
89
89
89
90
93
90
91
95
73
90
94
93
H
C3F7
C4F9
H(CF2)2
H(CF2)4
H(CF2)6
CF3
H
96
H
94
H
92
H
95
g
h
i
Ph
Me
Et
H
>98
93
CF3
CF3
97
j
C6F5
C6F5
CF3
>98
82
k
I
Me
>98
>98
>98
m
n
CF3
CF3
Scheme 3. Continuous-flow reaction conditions for biomimetic reductive
amination using DBU as a base.
whole silica gel column volume. Thus assembled column should
have the following optimal relative ratios between the three
functional parts: protection zone (about 1/4 of the volume),
reaction zone (1/4) and purification zone (1/2). A series of various
fluorine-containing imines 14a–n, prepared by standard proce-
dures [11b], were eluted through the column using a mixture of
hexanes/acetonitrile (4/1) as a solvent. Expectedly, the rate of the
desired isomerization of imines 14a–n to products 15a–n was
found to depend heavily on concentration of the starting
compounds, amount of DBU and rate of the elution. Optimization
of these parameters indicated that for a given amount of the
catalyst (see Section 1) the imine concentration of about 10 mol%
and the elution rate about 1 drop a second from the column tip,
generally provide for complete isomerization of starting imines
14a–n to products 15a–n. The structure and purity of the products
15a–n, conveniently collected from the column’s tip, were
determined by NMR.
Taking into account that the nature of starting imine 14a–n has
a significant effect on the isomerization rate, the practical rate of
the elution and/or the imine 14a–n concentration should be
experimentally adjusted for each particular case. However, there
are some useful generalizations can be used as, for instance, the
group of fluoroalkyl aldehydes 14a–f and ketones 14l–n derived
imines showed similar reactivity and therefore can be successfully
isomerized under the same set of reaction conditions. Another
group of compounds with similar reactivity are ketones derived
imines 14g–i,k and imine 14j, prepared from perfluorobenzalde-
hyde. Their isomerization to the corresponding Schiff bases 15g–k
is relatively slow, therefore lower elution rate/lower concentra-
tion/longer reaction zone should be used. Experimentally it was
found that the twice lower, as compared with the group of 14a–f,
14l–n imines, elution rate is generally sufficient to provide
complete transformation of 14g–k to the compounds 15g–k.
The structure and purity of the products 15a–n were confirmed by
NMR and compared with the literature data [11,12,14].
high and can be estimated as 5 mol% for about preparation of 1 mol
of the products. For repeated use of the same column the elution
rate should be slowed by 10% for each 15 mol% of product. The
decrease of catalytic activity is most probably the result of CO2
intake form the air as well as some partial (<1%) dehydrofluorina-
tion after products 15 formation. No attempts have been made to
regenerate the activity of the catalyst.
With these results in hand we explored next the asymmetric
biomimetic reductive amination under continuous-flow reaction
conditions. As shown in Scheme 4, the conventional procedure
[16,17] for asymmetric biomimetic transamination includes
condensation of ketones 16 with chiral amine 17 to form imines
18 followed by their DBU-catalyzed isomerization to Schiff bases
19, which can be easily hydrolyzed to the target amines 20 and
acetophenone as a byproduct.
The key step of this process, the base-catalyzed isomerization of
18 to 19, is notably stereoselective (up to 97% ee) regardless that
the products 19 are substantially less configurationally stable as
compared with starting compounds 18. However, the relative
configurational instability of products 19 creates a few problems in
the conventional in-flask process. Thus, compounds 19 can
undergo some racemization, especially in the presence of relatively
strong base, like DBU. For example, in the isomerization of 18
(R = Ph, CF = CF3) to highly C–H acidic product 19, the stereochem-
ical outcome can range from 50 to 87% ee [17]. In principle, the
partial racemization problem cannot be solved under the
conventional ‘‘in-flask’’ conditions as the products must be
exposed to the action of base until the reaction completion. On
the other hand, the continuous-flow reaction conditions could
offer a unique technical solution to this synthetic complication as
the corresponding products can be gradually removed from the
reaction zone virtually immediately upon their formation.
To realize the asymmetric reductive amination process under
the continuous-flow reaction condition, the same design (Scheme
3) can be used. The only difference in this case is a necessity to run
the process under elevated temperature as the isomerization of
chiral derivatives 21a–g to imines 22a–g takes place under
relatively forced reaction conditions. Therefore, we need to add the
‘‘heating zone’’ unit, which can be easily implemented using a
usual heating spiral or a mantle (set at 50 8C), covering completely
the ‘‘reaction zone’’ and about half of the ‘‘protection zone’’. Imines
21a–g were charged onto top of the column as described above for
the achiral process. The rate of elution vs. completion of the
isomerization (19F NMR) was the key issue to limit, to the very
minimum possible, the time of the products 22a–g exposure to the
DBU in the ‘‘reaction zone’’. After some experimentations we found
The data collected (Table 1) clearly demonstrates that the
isomerization of ket- and aldimines 14a–n to the products 15a–n,
under the continuous-flow reaction conditions (on-column
process) is generally more synthetically efficient, as compared
with a conventional (in-solution) reactions. Remarkably, in all
cases studied the Schiff bases 15a–n were obtained in higher yields
and of sufficient purity, allowing their further hydrolysis to the
target amines without any additional purification. Furthermore,
the overall experimental procedure is also significantly more
operationally convenient. The durability of the catalyst (SiO2-
absorbed DBU) used in this continuous-flow reaction procedure is

264
V.A. Soloshonok et al. / Journal of Fluorine Chemistry 131 (2010) 261–265
Scheme 4. General scheme for in-flask asymmetric biomimetic reductive amination of fluorinated carbonyl compounds.
that, for the indicated above amount of DBU and the imines 21a–e
concentration, the elution rate about 1 drop per three seconds
provided for >95% conversion of starting compounds 21a–e to the
Triazabicyclo[4.4.0]dec-1-ene (TBD) [24] can be recommended to
study.
imines 22a–e. For isomerization of b-amino acid derivatives 21f,g
1. Experimental part
the same elution rate can be used, however the temperature in the
‘‘heating zone’’ should be raised to 70 8C. The structure and purity
of the products 22a–g were confirmed by NMR and compared with
the literature data [16,17]. Taking into account that fluorine-
containing compounds have a tendency for high magnitude of the
self-disproportionation of enantiomers via ap-chromatography
[22] of evaporation/sublimation [23] the enantiomeric composi-
tion of products 22a–g was determined directly on the samples
collected from the column before even evaporation of the solvent,
using SUMICHIRAL OA-4500; eluent: n-hexane/dichloromethane/
ethanol = 60/30/10.
As it follows from the data presented in Table 2, the
stereochemical outcome of the isomerization reactions conducted
under the continuous-flow conditions, was generally higher as
compared with that of in-flask reactions [16,17]. For example, in the
case of preparation of the most C–H acidic product 22a, the
enantioselectivity was improved from 77 (in-flask) to 93% ee. In the
caseofderivative22b, trifluoromethylandcontainingbenzylgroups,
the yield and enantioselectivity were both improved. In the case of
the most unstable (prone to dehydrofluorination) product 22e
noticeable increase in chemical yield was observed (from 74 to 87%).
The structure and purity of the products 15a–n [11,12,14] and
22a–g [16,17] were confirmed by NMR and compared with the
literature data. The enantiomeric composition of compounds 22a–
g was determined using SUMICHIRAL OA-4500; eluent: n-hexane/
dichloromethane/ethanol = 60/30/10.
General procedure for isomerization of 14a–n to 15a–n. In
typical experiment, a glass column (3 cm ꢀ 80 cm) was charged
with 200 g of silica gel (hexanes). While leaving about 1 cm of the
solvent on top of silica gel, a suspension of 100 g of silica gel in
dichloromethane containing 1 g of DBU was charged onto the top
and allowed to percolate down to the surface of silica gel. Finally,
additional 100 g of silica gel (hexanes) was added on the top of the
column allowing the solvent to percolate down to the surface of
silica gel completely. Using thus prepared column, a solution
[10 mol% in hexanes/acetonitrile (4/1)] of fluorinated imine 14a–n
was eluted through the column at the rate of 1 drop per second.
UV-active fractions (detection by TLC) were collected and
evaporated to afford products 15a–n (for yields, see Table 1).
General procedure for isomerization of 21a–g to 22a–g. Typical
set-up of the column and experiment: a glass column (3.9 cm ꢀ
104 cm) was charged with 260 g of silica gel (hexanes). While
leaving about 1 cm of the solvent on top of silica gel, a suspension of
130 g of silica gel in dichloromethane containing 1.3 g of DBU was
charged onto the top and allowed to percolate down to the surface of
silica gel. Finally, additional 150 g of silica gel (hexanes) was added
on the top of the column allowing the solvent to percolate down to
the surface of silicagel completely. A heatingmantlewas attached to
the column covering fully the corresponding ‘‘reaction zone’’ and
about halfofthe‘‘protectionzone’’; thetemperaturewas set at50 8C.
Using thus prepared column, a solution [10 mol% in hexanes/
acetonitrile(4/1)]offluorinatedimine21a–gwaselutedthroughthe
column at the rate of 1 drop per second. UV-active fractions
(detection by TLC) were collected and evaporated to afford products
22a–g (for yields and enantioselectivity, see Table 2).
b-Amino acid derivatives 21f,g were also isomerized to products
22f,g with a bit better chemical yields and stereoselectivity.
In summary, the results presented here clearly demonstrate
that the continuous-flow conditions (on-column) for biomimetic
transamination have noticeable advantage of the conventional in-
flask procedure in terms of chemical yield, stereochemical
outcome and operational convenience. In particular, this new
approach may be recommended for repeating, large-scale synthe-
sis of biologically important fluorinated amines and amino acids.
As the next step in the further development of continuous-flow
methodology application of polymer-bound bases, such as 1,5,7-
Table 2
Yields and stereochemical outcome for isomerization of 21a–g to 22a–g under the
conditions of continuous-flow reaction.
Acknowledgements
21
R
RF
22
On-column
In-flask
This work was supported by the Department of Chemistry and
Biochemistry, University of Oklahoma. The authors gratefully
acknowledge generous financial support from Central Glass
Company (Tokyo, Japan) and Ajinomoto Company (Tokyo, Japan).
yield (%)/% ee
(lit. data)
yield (%)/% ee
a
b
c
Ph
Bn
Me
Et
CF3
CF3
CF3
CF3
C3F7
95/93
93/91
95/93
92/91
87/95
98/77
86/88
94/93
87/87
74/97
References
d
e
Me
[1] (a) S.-I. Murahashi, D. Zhang, Chem. Soc. Rev. 37 (2008) 1490–1501;
(b) I. Navarro, J.-F. Basset, S. Hebbe, S.M. Major, T. Werner, C. Howsham, J.
Brackow, A.G.M. Barrett, J. Am. Chem. Soc. 130 (2008) 10293–10298;
(c) Y.-S. Yao, Z.-J. Yao, J. Org. Chem. 73 (2008) 5221–5225;
f
CF3
92/90
90/85
g
CF3
91/95
98/93
(d) R. Rong, F.E. McDonald, Angew. Chem. Int. Ed. 47 (2008) 4377–4379;
(e) S.P. Visser, Chem. Eur. J. 14 (2008) 4533–4541;
(f) S.J. Eade, M.W. Walter, C. Byrne, B. Odell, R. Rodriguez, J.E. Baldwin, R.M.
Adlington, J.E. Moses, J. Org. Chem. 73 (2008) 4830–4839;

V.A. Soloshonok et al. / Journal of Fluorine Chemistry 131 (2010) 261–265
265
(g) M. Amat, R. Griera, R. Fabregat, E. Molins, J. Bosch, Angew. Chem. Int. Ed. 47
(2008) 3348–3351;
(b) V.A. Soloshonok, Y.L. Yagupolskii, V.P. Kukhar, Zh. Org. Khim. 24 (1988) 1638–
1644, Chem. Abstr. 110, 154827b.
(h) J. Piera, J.-E. Backvall, Angew. Chem. Int. Ed. 47 (2008) 3506–3523;
(i) R.A. Scheck, M.T. Dedeo, A.T. Iavarone, M.B. Francis, J. Am. Chem. Soc. 130
(2008) 11762–11770.
[11] (a) V.A. Soloshonok, A.G. Kirilenko, V.P. Kukhar, G. Resnati, Tetrahedron Lett. 35
(1994) 3119–3122;
(b) T. Ono, V.P. Kukhar, V.A. Soloshonok, J. Org. Chem. 61 (1996) 6563–6569;
(c) V.A. Soloshonok, T. Ono, Synlett (1996) 919–921;
(d) V.A. Soloshonok, T. Ono, Tetrahedron 52 (1996) 14701–14712;
(e) H. Ohkura, D.O. Berbasov, V.A. Soloshonok, Tetrahedron Lett. 44 (2003) 2417–
2420;
[2] (a) D. Boyal, D.E. Frantz, E.M. Carreira, Org. Lett. 4 (2002) 2605–2608;
(b) V.A. Soloshonok, H. Ohkura, A. Sorochinsky, N. Voloshin, A. Markovsky, M.
Belik, T. Yamazaki, Tetrahedron Lett. 43 (2002) 5445–5448;
(c) A. Sorochinsky, N. Voloshin, A. Markovsky, M. Belik, N. Yasuda, H. Uekusa, T.
Ono, D.O. Berbasov, V.A. Soloshonok, J. Org. Chem. 68 (2003) 7448–7454;
(d) W. Qiu, V.A. Soloshonok, C. Cai, X. Tang, V.J. Hruby, Tetrahedron 56 (2000)
2577–2582;
(f) V.A. Soloshonok, H. Ohkura, M. Yasumoto, J. Fluorine Chem. 127 (2006) 708–
711;
(g) M. Yasumoto, H. Ueki, V.A. Soloshonok, J. Fluorine Chem. 128 (2007) 736–739.
[12] (a) H. Ohkura, D.O. Berbasov, V.A. Soloshonok, Tetrahedron 59 (2003) 1647–
1656;
(b) D.O. Berbasov, I.D. Ojemaye, V.A. Soloshonok, J. Fluorine Chem. 125 (2004)
603–607;
(c) V.A. Soloshonok, D.O. Berbasov, J. Fluorine Chem. 125 (2004) 1757–1763.
[13] (a) V.A. Basiuk, T.Y. Gromovoy, A.A. Chuiko, V.A. Soloshonok, V.P. Kukhar, Syn-
thesis (1992) 449–451;
(e) V.A. Soloshonok, X. Tang, V.J. Hruby, Tetrahedron 57 (2001) 6375–6382;
(f) T.K. Ellis, C.H. Martin, G.M. Tsai, H. Ueki, V.A. Soloshonok, J. Org. Chem. 68
(2003) 6208–6214;
(g) J.L. Moore, S.M. Taylor, V.A. Soloshonok, ARKIVOC vi (2005) 287.
[3] A.E. Braunshtein, M.G. Kritsman, Biokhimiya (Moscow) 2 (1937) 859–874.
[4] (a) A.E. Martell, Acc. Chem. Res. 22 (1989) 115–124;
(b) M.E. Tanner, Acc. Chem. Res. 35 (2002) 237–246;
(c) G. Wachtershauser, Microbiol. Rev. 52 (1988) 452–484;
(d) M.D. Toney, J.F. Kirsch, Biochemistry 32 (1993) 1471–1479;
(e) J.B. Longenecker, E.E. Snell, J. Am. Chem. Soc. 79 (1957) 142–145;
(f) A.B. Khotkevich, V.A. Soloshonok, Y.L. Yagupolskii, Zh. Obshch. Khim. 60
(1990) 1005–1008, Chem. Abstr. 113, 171274y;
(b) V.A. Soloshonok, V.P. Kukhar, Tetrahedron 53 (1997) 8307–8314.
[14] (a) V.A. Soloshonok, A.G. Kirilenko, V.P. Kukhar, G. Resnati, Tetrahedron Lett. 34
(1993) 3621–3624;
(b) V.A. Soloshonok, A.G. Kirilenko, N.A. Fokina, I.P. Shishkina, S.V. Galushko, V.P.
Kukhar, V.K. Svedas, E.V. Kozlova, Tetrahedron: Asymmetry 5 (1994) 1119–1126;
(c) V.A. Soloshonok, A.G. Kirilenko, N.A. Fokina, S.V. Galushko, V.P. Kukhar, V.K.
Svedas, G. Resnati, Tetrahedron: Asymmetry 5 (1994) 1225–1228;
(d) V.A. Soloshonok, V.P. Kukhar, Tetrahedron 52 (1996) 6953–6964.
[15] V.A. Soloshonok, H. Ohkura, K. Uneyama, Tetrahedron Lett. 43 (2002) 5449–5452.
[16] (a) V.A. Soloshonok, T. Ono, I.V. Soloshonok, J. Org. Chem. 62 (1997) 7538–7539;
(b) V.A. Soloshonok, I.V. Soloshonok, V.P. Kukhar, V.K. Svedas, J. Org. Chem. 63
(1998) 1878–1884;
(g) P. Nagy, H. Ueki, D.O. Berbasov, V.A. Soloshonok, J. Fluorine Chem. 129 (2008)
409–415.
[5] (a) L. Liu, W. Zhou, J. Chruma, R. Breslow, J. Am. Chem. Soc. 126 (2004) 8136–
8137;
(b) L. Liu, R. Breslow, J. Am. Chem. Soc. 125 (2003) 12110–12111;
(c) R.F. Zabinski, M.D. Toney, J. Am. Chem. Soc. 123 (2001) 193–198;
(d) S. Ogo, K. Uehara, T. Abura, S. Fukuzumi, J. Am. Chem. Soc. 126 (2004) 3020–
3021;
(e) L. Casella, M. Gullotti, J. Am. Chem. Soc. 105 (1983) 803–809;
(f) L. Liu, M. Rozenman, R. Breslow, J. Am. Chem. Soc. 124 (2002) 12660–12661;
(g) L. Liu, R. Breslow, J. Am. Chem. Soc. 124 (2002) 4978–4979;
(h) R. Breslow, J.W. Canary, M. Varney, S.T. Waddell, D. Yang, J. Am. Chem. Soc.
112 (1990) 5212–5219;
(c) V.A. Soloshonok, H. Ohkura, M. Yasumoto, J. Fluorine Chem. 127 (2006) 924–
929;
(d) V.A. Soloshonok, H. Ohkura, M. Yasumoto, J. Fluorine Chem. 127 (2006) 930–
935.
[17] V.A. Soloshonok, T. Ono, J. Org. Chem. 62 (1997) 3030–3031.
[18] (a) V.A. Soloshonok, A.G. Kirilenko, S.V. Galushko, V.P. Kukhar, Tetrahedron Lett.
35 (1994) 5063–5064;
(b) V.A. Soloshonok, M. Yasumoto, J. Fluorine Chem. 128 (2007) 170–173.
[19] V.A. Soloshonok, M. Yasumoto, J. Fluorine Chem. 127 (2006) 889–893.
[20] (a) A.L. Upthagrove, W.L. Nelson, Drug Metabol. Dispos. 29 (2001) 1377–1388;
(b) J. Jiang, H. Shah, R.J. DeVita, Org. Lett. 5 (2003) 4101–4103;
(c) C. Yuan, S. Li, J. Xiao, S. Cui, G. Wang, Phosphorus Sulfur Silicon Relat. Element.
177 (2002) 1731–1734;
(i) D.A. Jaeger, M.D. Broadhurst, D.J. Cram, J. Am. Chem. Soc. 101 (1979) 717–732;
(j) R.W. Layer, Chem. Rev. 63 (1963) 489–510;
(k) P.A.S. Smith, C. Van Dang, J. Org. Chem. 41 (1976) 2013–2015;
(l) H. Brandenberger, P.P. Cohen, Helv. Chim. Acta 36 (1953) 549–554.
[6] E.J. Corey, K. Achiwa, J. Am. Chem. Soc. 91 (1969) 1429–1432.
[7] (a) V. Calo, L. Lopez, P.E. Todesco, J. Chem. Soc., Perkin Trans. 1 (1972) 1652–1653;
(b) S. Ohta, M. Okamoto, Synthesis (1982) 756–758;
(c) J.H. Babler, B.J. Invergo, J. Org. Chem. 46 (1981) 1937–1938;
(d) T.F. Buckley, H. Rapoport, J. Am. Chem. Soc. 104 (1982) 4446–4450;
(e) F.G. Baddar, Z. Iskander, Nature 167 (1951) 1069–1070;
(f) F.G. Baddar, J. Chem. Soc. (1950) 136–139;
(d) C. Yuan, S. Li, J. Xiao, Heteroatom Chem. 11 (2000) 541–545;
(e) J. Xiao, X. Zhang, C. Yuan, Heteroatom Chem. 11 (2000) 536–540;
(f) V. Michaut, F. Metz, J.M. Paris, J.C. Plaquevent, J. Fluorine Chem. 128 (2007)
500–506;
(g) F.G. Baddar, Z. Iskander, J. Chem. Soc. (1954) 203–209;
(h) F.G. Baddar, Z. Iskander, J. Chem. Soc. (1954) 209–213.
[8] (a) G. Cainelli, D. Giacomini, A. Trere, P.P. Boyl, J. Org. Chem. 61 (1996) 5134–
5139;
(g) V. Michaut, F. Metz, J.M. Paris, J.C. Plaquevent, J. Fluorine Chem. 128 (2007)
889–895.
[21] (a) V.A. Soloshonok, T. Ono, J. Fluorine Chem. 129 (2008) 785–787;
(b) V.A. Soloshonok, H.T. Catt, T. Ono, J. Fluorine Chem. 130 (2009) 512–515.
[22] (a) V.A. Soloshonok, Angew. Chem. Int. Ed. 45 (2006) 766–769;
(b) V.A. Soloshonok, D.O. Berbasov, J. Fluorine Chem. 127 (2006) 597–603;
(c) V.A. Soloshonok, D.O. Berbasov, Chim. Oggi/Chem. Today 24 (2006) 44–47.
[23] V.A. Soloshonok, H. Ueki, M. Yasumoto, S. Mekala, J.S. Hirschi, D.A. Singleton, J.
Am. Chem. Soc. 129 (2007) 12112–12113.
(b) J.G.H. Willems, J.G. de Vries, R.J.M. Nolte, B. Zwanenburg, Tetrahedron Lett. 36
(1995) 3917–3920;
(c) A. Hjelmencrantz, U. Berg, J. Org. Chem. 67 (2002) 3585–3594;
(d) S. Bachmann, K.R. Knudsen, K.A. Jørgensen, Org. Biomol. Chem. 2 (2004)
2044–2049;
(e) Y. Wu, P. Ahlberg, J. Org. Chem. 57 (1992) 6324–6327;
(f) Y. Wu, P. Ahlberg, Acta Chem. Scand. 46 (1992) 60–72;
(g) K. Bernauer, R. Deschenaux, T. Taura, Helv. Chim. Acta 66 (1983) 2049–2058;
(h) R. Deschenaux, K. Bernauer, Helv. Chim. Acta 67 (1984) 373–377.
[9] M. Ando, H. Kuzuhara, Bull. Chem. Soc. Jpn. 63 (1990) 1925–1928.
[10] (a) V.A. Soloshonok, I.I. Gerus, Y.L. Yagupolskii, V.P. Kukhar, Zh. Org. Khim. 24
(1988) 993–997, Chem. Abstr. 110, 134824v;
[24] (a) R.I. Storer, T. Takemoto, P.S. Jackson, D.S. Brown, I.R. Baxendale, S.V. Ley,
Chem. Eur. J. 10 (2004) 2529–2547;
(b) F. Fringuelli, F. Pizzo, C. Vittoriani, L. Vaccaro, Chem. Commun. (2004) 2756–
2757;
(c) D. Simoni, R. Rondanin, M. Morini, R. Baruchello, F.P. Invidiata, Tetrahedron
Lett. 41 (2000) 1607–1610;
(d) W. Ye, J. Xu, C.T. Tan, C.H. Tan, Tetrahedron Lett. 46 (2005) 6875–6878.